Many of the materials used in the manufacture of semiconductors, optics and a variety of other applications require the highest quality material available to meet the performance requirements expected in the future. This is particularly important when it relates to the quality and cleanliness of the surface, and the crystalline structure, defects and impurities in the material. In the case of semiconductors like silicon and gallium arsenide for instance, crystalline defects or impurities on or near the surface of the material can significantly degrade the performance of electronic components and integrated circuits made from that material, or keep them from operating at all. Material defects in optics made from semiconductors, glasses and metals can have catastrophic results when used with high powered lasers or when second order optical effects are being used. These situations have been recognized for some time, and a variety of equipment has been disclosed or developed to measure the surface character of these special materials. For example U.S. Pat. No. 4,314,763, entitled "DEFECT DETECTION SYSTEM", discloses one of several techniques used to measure surface defects and contamination on semiconductors.
The measurement of the crystalline and other micro defects directly below the surface, however, has been much more difficult. For example U.S. Pat. No. 4,391,524, entitled "METHOD FOR DETERMINING THE QUALITY OF LIGHT SCATTERING MATERIAL", similar to the one previously mentioned, discloses one technique developed for that purpose. A second approach is described in U.S. Pat. No. 4,352,016, entitled "METHOD AND APPARATUS FOR DETERMINING THE QUALITY OF A SEMICONDUCTOR SURFACE" and U.S. Pat. No. 4,314,017, entitled "APPARATUS FOR DETERMINING THE QUALITY OF A SEMICONDUCTOR SURFACE". All of these measurement techniques have significant limitations when measuring subsurface crystalline damage and other micro defects, both surface and subsurface, which are most important to the improvement and use of these materials.
The term defects, as used herein, refers to any of a variety of structural crystalline defects found near the surface in bulk material or in layers or thin films, either grown-in or processing induced, like slips, dislocations, stacking faults and even buried scratch traces as well as defects which are formed when foreign material is incorporated into the crystal structure such as inclusions, precipitates and impurity clusters and other impurity related defects. Likewise non-crystalline structural defects that occur in amorphous and polycrystalline materials such as voids, buried scratches and pits, and the interface between layers either purposely deposited or formed during processing are included. The term defects is also used to describe surface features such as pits, scratches, scuffs, pinholes and exposed impurity clusters, inclusions and bubbles as well as particles and other surface contamination.
Basically, there are three ways of generating defects in the materials of interest. First, defects can be incorporated into the material when it is manufactured in its bulk form. For instance, when single crystals of silicon or gallium arsenide are grown, dislocations can form in the boule due to thermal stresses induced during the growing process, or when impurities in the starting material, and from other sources, are incorporated into the crystal. For polycrystalline and amorphous materials, impurities can easily come from the starting materials, the preparation tools that come in contact with the material and even the gases in the environment where the materials are being made. Bubbles and inclusions are also formed during the melting and cooling process.
Secondly, after the material is manufactured, it must be cut into usable pieces and the surfaces ground and polished in preparation for further processing. These steps of cutting, grinding and polishing also introduce slips, dislocations and more impurities into the crystal structure just below the prepared surface as well as surface defects like pits and scratches. Polycrystalline and amorphous materials have the same problems with surface defects. They can also have subsurface defects caused by the high pressures used in grinding and polishing. These buried defects form as part of a layer of material at the surface, which under pressure has either recrystallized or become amorphous, and the bulk of the material. Impurities can also be introduced into the material during these operations by diffusion and other mechanisms. In the case of semiconductors, this second class of defects, those that are processing induced, is generally 1,000 to 1,000,000 times greater in number than the defects grown into the original boule of material. Not only are the numbers larger, but as stated, these defects are all located near the surface while the grown-in defects are distributed throughout the volume of the material.
Thirdly, defects such as stacking faults, precipitates, dislocation lines and ion implantation induced defects can be generated by various fabrication processes typically used in the processing of semiconductor wafers. The same is true of optical and other materials not only for crystalline defects but also for the buried defects which can be generated in amorphous and polycrystalline materials by the preparation processes. Fabrication processes such as coating, etching, ion implantation and cleaning can all cause surface and subsurface defects. These defects will affect the way light is transmitted through or reflected from an optical material and can affect the properties of electronic materials. Another effect, which is just beginning to be understood, is the connection between subsurface defects of all types and the defects in coatings deposited on the surface. It has long been known that substrate surface defects can cause flaws in the thin film coatings placed on them. Subsurface defects are more subtle in that they are very difficult to detect nondestructively but can be equal to surface defects in causing flaws in the coatings. Since optics, electronics and many other applications make extensive use of coatings, such effects are of great importance. For instance, epitaxial layers grown on semiconductor wafers can have stacking faults grown-in during the manufacturing process and these can be related to the defects already existing in the substrate wafer.
One technique currently used to measure crystalline damage is described in U.S. Pat. No. 4,352,016 and No. 4,352,017. This approach measures the reflectance of ultraviolet light, at two wavelengths, from the surface of a semiconductor wafer. This technique is known to be insensitive to damage at any depth in the material primarily because of the use of ultraviolet light which is a shallow penetrator in semiconductor materials. A second factor significantly limiting sensitivity is the reflectance measurement itself. Such measurements are notoriously difficult to make and result in looking for small variations in large numbers, which is one of the reasons why this technique requires measurements at two wavelengths. The practical application of this reflectance technique shows up these deficiencies.
A second approach is described in U.S. Pat. No. 4,391,524. This approach can measure the light scattered from the surface and subsurface regions but because of the geometry of the measurement, important data is lost. There are three factors which bear on this assessment which are independent of the wavelength selected for the probe beam. First, the angle of incidence of the probe beam is 0.degree.. This eliminates any possibility of determining the directional nature of the defects, or of using polarization to help discriminate between surface and subsurface defects. Secondly, the detector subtends a large solid angle thus integrating scatter from all directions, again making impossible the determination of directional defects, and at the same time diluting the signature of the defects it is designed to measure. And finally, the detector line of sight is also at 0.degree., or near 0.degree.. This introduces significant amounts of surface scatter into the measured signal which is nearly impossible to separate from the subsurface scatter under these conditions. Subtle variations in surface scatter will mask the scatter from the subsurface that are the purpose of the measurement. The result is a measurement that is insensitive to oriented defects, which most subsurface defects are, and even insensitive to many very small surface defects which are also oriented.
Other techniques which purport to measure surface and/or subsurface defects using a scatter measurement technique all measure the total integrated scatter from the surface of the test part. This technique known as TIS integrates the scatter from the surface and subsurface as well as from all directions. The result is an insensitive measurement of mostly surface roughness for which this technique was originally designed. The surface scatter component of the total scatter from a material is very large, and will overwhelm the subsurface component if the scattered light is collected anywhere near the specularly reflected beam. Also, many very small surface defects such as pits and particles are faceted so that the scatter is generally large in one direction and nonexistent in all the others. For large pits and particles the integration that takes place does not dilute the signal, but for the very small pits and particles, the integrated signal from TIS will not show a noticeable change.
All of the scatter measurement techniques available for the nondestructive detection of surface defects are limited in that they are not able to accurately detect defects below about 1 micron in size unless the defects are uniformly shaped, spherical or hemispherical. Real defects, especially particles, are not uniformly shaped but are faceted or oblong or are some unusual shape. These odd shaped defects do not scatter light uniformly and can not be accurately detected by such techniques. In fact, only a small fraction of such defects can be detected by these techniques.